Coated sand, manufacturing method for same, and manufacturing method for mold

ABSTRACT

Purposes of the present invention are to provide a dry coated sand having a high degree of fluidity at the room temperature, a method of advantageously producing the coated sand, and a method of producing a casting mold having excellent properties, by using the coated sand. The dry coated sand is obtained by mixing an aqueous solution of a water glass used as a binder, with a heated refractory aggregate, whereby water in the aqueous solution is evaporated, and a coating layer of the binder is formed on surfaces of the refractory aggregate. A moisture percentage in the thus obtained dry coated sand is controlled so as to be not more than 0.5% by mass. The intended casting mold is obtained by filling a molding cavity of a forming mold, with the dry coated sand, and passing a steam through the coated sand, to solidify or cure the coated sand.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of the International Application No. PCT/JP2013/083903, filed on Dec. 18, 2013, which claims the benefit under 35 U.S.C. §119(a)-(d) of Japanese Application No. 2012-276478, filed on Dec. 19, 2012, the entireties of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a coated sand, a method of producing the same, and a method of producing a casting mold, and more particularly to a coated sand which is in a dry state and which has fluidity at the room temperature, a method of producing the coated sand, and a method of producing a casting mold by using the coated sand.

2. Description of Related Art

As one type of a casting mold used for casting a molten metal, a casting mold obtained by forming a coated sand into a desired shape has been used. The coated sand used for the casting mold is obtained by coating a molding sand consisting of a refractory aggregate, with a suitable binder. As examples of the binder, inorganic binders such as a cement and a water glass, and organic binders such as a phenolic resin, a furan resin and a urethane resin are disclosed, together with methods of forming self-curing molds by using these binders, on pages 78-90 of Chuzou Kougaku Binran (Handbook of Foundry Engineering) edited by Japan Foundry Engineering Society.

JP-A-2009-90334 discloses a method of producing the casting mold by using a resin-coated sand obtained by coating the molding sand with a thermosetting resin such as the phenolic resin, which is one of the above-indicated organic binders. In this method, the intended casting mold is produced by the following steps: filling a forming mold with the resin-coated sand; blowing a steam into the forming mold to raise a temperature of the resin-coated sand; and then blowing a heated gas into the forming mold to evaporate condensation water within the forming mold and to heat the binder of the resin-coated sand to a temperature not lower than a temperature at which the binder is solidified or cured. However, in this method, the phenolic resin or other resin is used as the binder, so that the binder may be decomposed at a high temperature at the time of formation of the casting mold or casting of the molten metal, giving rise to a problem of generation of gases due to decomposition of phenol and aldehyde. Odors and stimulants of these gases cannot be completely eliminated, so that the above-described method is not suitable for applications in which the odors and stimulants should be avoided.

On the other hand, where the water glass is used as the inorganic binder, it is necessary to use a curing agent such as a CO₂ gas to cure the water glass. In the conventional technique of such a water glass/CO₂ gas process, the molding sand (refractory aggregate) is kneaded with an aqueous solution of the water glass used as the binder, to coat surfaces of the molding sand with the binder, and the casting mold is formed by using the thus obtained coated sand in a wet state (a moist state) in which the wet water glass adheres to the surfaces of the molding sand. Such a coated sand has a low degree of fluidity, so that there arise inherent problems of difficulty in filling the forming mold with the coated sand, occurrence of filling defects, and low productivity of the casting mold.

Under the above-described circumstances, in order to obtain a binder-coated refractory (coated sand) having a high degree of fluidity, JP-A-2012-76115 proposes to use, as the binder, a water-soluble inorganic compound selected from a group consisting of the water glass, sodium chloride, sodium phosphate, sodium carbonate, sodium vanadate, sodium borate, aluminum sodium oxide, potassium chloride and potassium carbonate. The binder-coated refractory proposed in this publication is obtained by coating surfaces of the refractory aggregate with a solid coating layer containing the water-soluble inorganic compound described above. This publication further discloses a method of producing the casting mold, which includes steps of: filling the forming mold with the binder-coated refractory; blowing a steam into the forming mold to heat the binder-coated refractory and to moisten the binder constituting the coating layer; and then solidifying the binder.

However, the inventor of the present invention studied the binder-coated refractory described above, and found that even where the surfaces of the refractory aggregate are coated with the solid coating layer of the binder consisting of the water-soluble inorganic compound such as the water glass and sodium chloride, a sufficiently high degree of fluidity of the binder-coated refractory cannot be necessarily secured, and the casting mold obtained by using the binder-coated refractory does not have a sufficiently high degree of strength. Further, in order to coat the refractory aggregate with the water-soluble inorganic compound (binder), the water-soluble inorganic compound is dissolved in water, and the thus obtained aqueous solution is used to coat the refractory aggregate. In this respect, it was found that conditions of the surfaces of the refractory aggregate coated with the water-soluble inorganic compound vary depending on a water content in the aqueous solution, so that physical properties of the binder-coated refractory vary depending on the water content in the aqueous solution. Namely, where the water content is excessively small, the refractory aggregate cannot be uniformly coated with the water-soluble inorganic compound. On the other hand, where the water content is excessively large, the binder-coated refractory cannot be sufficiently dried. Therefore, the use of the binder-coated refractory gives rise to problems that formability and the physical properties of the binder-coated refractory are not satisfactory or have undesirable variations. Further, the properties of the binder-coated refractory considerably vary depending on the kind of the water-soluble inorganic compound, so that where the binder-coated refractory is produced by using different kinds of the water-soluble inorganic compounds, under the same conditions, the physical properties of the binder-coated refractory have undesirable variations, giving rise to an inherent problem of difficulty in optimizing the conditions of its production.

SUMMARY OF THE INVENTION

The present invention was made based on the background art described above. Therefore, objects of the present invention are to provide: a coated sand which is in a dry state and which has fluidity at the room temperature; a method of advantageously producing the coated sand; and a method of producing a casting mold having excellent properties by using the coated sand. Other objects of the present invention are to provide: a coated sand which permits a considerable improvement in ease of filling of a molding cavity of a forming mold used for producing a casting mold, with the coated sand, and a further improvement of a strength of the obtained casting mold; a method of producing the coated sand; and a method of producing the casting mold by using the coated sand.

In order to achieve the above-described objects, the present invention can be preferably embodied in various modes which will be described below. The various modes of the invention described below may be practiced in any combination. It is to be understood that the modes and technical features of the present invention are not limited to those described below, and can be recognized based on the inventive concept disclosed in the specification taken as a whole.

(1) A coated sand which is in a dry state and which has fluidity at the room temperature, the coated sand being obtained by mixing an aqueous solution of a water glass used as a binder, with a heated refractory aggregate, to evaporate water in the aqueous solution of the water glass, for thereby forming a coating layer of the binder on surfaces of the refractory aggregate, the coated sand being characterized in that its moisture percentage is controlled so as to be not more than 0.5% by mass.

(2) The coated sand according to the above-described mode (1), characterized in that the coated sand includes not more than 3% by mass of lumps which do not pass through a 20-mesh screen.

(3) The coated sand according to the above-described mode (1) or (2), characterized in that the aqueous solution of the water glass is an aqueous solution of an alkali metal silicate.

(4) The coated sand according to the above-described mode (3), characterized in that the alkali metal silicate has a molar ratio of silicon dioxide to an alkali metal oxide, which molar ratio is not smaller than 1.0 and smaller than 3.0.

(5) The coated sand according to the above-described mode (3) or (4), characterized in that the alkali metal silicate is sodium silicate.

(6) The coated sand according to the above-described mode (5), characterized in that the sodium silicate has a molar ratio SiO₂/Na₂O of not smaller than 1.0 and smaller than 3.0.

(7) The coated sand according to any one of the above-described modes (1) to (6), characterized in that a nonvolatile content in the aqueous solution of the water glass is 20-45% by mass.

(8) The coated sand according to any one of the above-described modes (1) to (7), characterized in that the aqueous solution of the water glass is used in an amount of 0.1-2.5 parts by mass, in terms of its solid content, with respect to 100 parts by mass of the refractory aggregate.

(9) A method of producing the coated sand according to any one of the above-described modes (1) to (8), characterized in that the refractory aggregate and the aqueous solution of the water glass are mixed together such that the water in the aqueous solution of the water glass is evaporated within 5 minutes after addition of the aqueous solution of the water glass to the refractory aggregate, to obtain the coated sand having the moisture percentage of not more than 0.5% by mass.

(10) A method of producing a casting mold, characterized in that the casting mold is obtained by filling a molding cavity of a forming mold which gives the casting mold, with the coated sand according to any one of the above-described modes (1) to (8), and then passing a steam through the coated sand, to solidify or cure the coated sand within the forming mold.

(11) The method of producing the casting mold according to the above-described mode (10), characterized in that a dry air, a heated dry air, a nitrogen gas or an argon gas is further passed through a filling phase of the coated sand filling the molding cavity of the forming mold, simultaneously with passing the steam through the coated sand.

(12) The method of producing the casting mold according to the above-described mode (10) or (11), characterized in that a dry air, a heated dry air, a nitrogen gas or an argon gas is further passed through a filling phase of the coated sand filling the molding cavity of the forming mold, after passing the steam through the coated sand.

(13) The method of producing the casting mold according to any one of the above-described modes (10) to (12), characterized in that at least one of a carbon dioxide gas, an ester gas and a carbonate gas is passed through a filling phase of the coated sand filling the molding cavity of the forming mold, simultaneously with or after passing the steam through the coated sand.

(14) The method of producing the casting mold according to any one of the above-described modes (10) to (13), characterized in that a pressure within the molding cavity of the forming mold is reduced before passing the steam through the coated sand.

(15) The method of producing the casting mold according to any one of the above-described modes (10) to (14), characterized in that the coated sand is preheated to a temperature not lower than 30° C., and then the molding cavity of the forming mold is filled with the preheated coated sand.

(16) The method of producing the casting mold according to any one of the above-described modes (10) to (15), characterized in that the forming mold is preheated and kept at an elevated temperature.

(17) A method of producing a casting mold, characterized in that the casting mold is formed by multilayer molding using the coated sand according to any one of the above-described modes (1) to (8).

In the present invention, the water glass is used as the binder, and the aqueous solution of the water glass is used to coat the refractory aggregate. By evaporating the water in the thus formed coating layer of the binder on the surfaces of the refractory aggregate, the coated sand is obtained in the dry state, such that the entirety of the coated sand has the moisture percentage of not more than 0.5% by mass. Thus, it is possible to further improve the fluidity of the coated sand, and considerably improve the ease of filling of the molding cavity of the forming mold used for producing the casting mold, with the coated sand, to advantageously produce the sound casting mold having a high degree of strength.

The strength of the casting mold obtained by using the coated sand can be further improved by reducing the amount of the lumps included in the coated sand, according to the preferable mode of the present invention. Further, by controlling the nonvolatile content in the aqueous solution of the water glass so as to be a low value, the water glass having a high concentration can be diluted with the water, and the thus obtained aqueous solution of the water glass can be used to efficiently and uniformly coat the refractory aggregate with the water glass. Thus, it is possible to advantageously obtain the coated sand including only a small amount of the lumps, and more advantageously improve the strength of the casting mold obtained by using the coated sand.

The coated sand according to the present invention is obtained by using the water glass as the binder. Accordingly, unlike the conventional resin coated sand obtained by using the organic binders such as the phenolic resin and the furan resin, the coated sand of the present invention can reduce or prevent generation of a gas component which has a low molecular weight and emits an odor, at the time of formation of the casting mold and casting of the molten metal. Therefore, the coated sand of the present invention has an advantage that its use does not result in generation of a gas, tar, odor and the like, and does not give rise to a problem of deterioration of production environment.

DETAILED DESCRIPTION OF THE INVENTION

A coated sand according to the present invention is obtained by mixing an aqueous solution of a water glass used as a binder, with a heated refractory aggregate, and evaporating water in the thus obtained mixture, in other words, evaporating the water contained in the aqueous solution of the water glass, for thereby forming a dry coating layer consisting of the water glass which serves as the binder, on surfaces of the refractory aggregate. The coated sand is in a dry state and has a sufficiently high degree of fluidity at the room temperature. In the present invention, a moisture percentage of the coated sand is controlled so as to be not more than 0.5% by mass, and advantageously not more than 0.3% by mass. It is recognized that the moisture percentage is preferably close to zero as far as possible. The coated sand which is provided with the coating layer of the water glass and which has the extremely low moisture percentage is used in the dry state in the absence of water, so that the coated sand flows smoothly and has excellent properties such as the sufficiently high degree of fluidity at the room temperature. Accordingly, it is possible to effectively improve ease of filling of a molding cavity of a forming mold used for producing a casting mold, with the coated sand, to advantageously obtain a sound casting mold and effectively improve a strength of the casting mold.

The coated sand according to the present invention preferably includes only a small amount of composite particles so-called lumps, each of which is formed of a plurality of particles combined with each other, and which are generated during a production process of the coated sand. Generally, it is recommended that where the coated sand obtained by the production process is sieved with a 20-mesh screen, an amount of the coated sand which does not pass through the 20-mesh screen, namely, an amount of the lumps left on the 20-mesh screen is not more than 3% by mass, and more preferably not more than 1% by mass, with respect to a whole amount of the coated sand. An excessively large amount of the lumps deteriorates the ease of filling of the molding cavity of the forming mold used for producing the casting mold, with the coated sand, giving rise to problems that the produced casting mold is likely to have defects, and the strength of the casting mold is difficult to be improved.

The refractory aggregate of the coated sand is a refractory material which serves as a base material of the casting mold. Any one of various refractory particulate materials conventionally used for the casting mold may be used as the refractory aggregate. Specific examples of the refractory aggregate include: a silica sand; a regenerated silica sand; special sands such as an alumina sand, an olivine sand, a zircon sand and a chromite sand; slag particles such as a ferrochromium slag, a ferronickel slag and a converter slag; artificial particles such as alumina particles and mullite particles, and regenerated particles thereof; an alumina ball; and a magnesia clinker. The above-indicated refractory aggregates may be: a new or fresh sand; a regenerated or reclaimed sand which has been used once or a plurality of times as a molding sand to form the casting mold; or a mixture of the regenerated or reclaimed sand and the new or fresh sand. The refractory aggregate used in the present invention generally has a grain size of about AFS 40-80, and preferably not larger than about AFS 60 in order to make it easy to pass a steam through the coated sand and dry the coated sand in formation of the casting mold.

The water glass used as the binder of the coated sand according to the present invention is a soluble silicate compound, and preferably an aqueous solution of an alkali metal silicate, such as sodium silicate, potassium silicate, sodium metasilicate, potassium metasilicate, lithium silicate, ammonium silicate, colloidal silica and alkyl silicate. It is possible to use a mixture of a plurality of kinds of the alkali metal silicates. Among various kinds of the alkali metal silicates, those having a molar ratio of silicon dioxide to an alkali metal oxide, which molar ratio is not smaller than 1.0 and smaller than 3.0, are preferably used. In the present invention, sodium silicate (silicate of soda) is advantageously used, since the coated sand obtained by using sodium silicate is not likely to suffer from blocking and has a high degree of formability. Commercially available sodium silicates are generally classified into No. 1 to No. 5 based on their SiO₂/Na₂O molar ratios. Specifically described, the sodium silicate No. 1 has the molar ratio SiO₂/Na₂O within a range between 2.0 and 2.3, the sodium silicate No. 2 has the molar ratio SiO₂/Na₂O within a range between 2.4 and 2.5, the sodium silicate No. 3 has the molar ratio SiO₂/Na₂O within a range between 3.1 and 3.3, the sodium silicate No. 4 has the molar ratio SiO₂/Na₂O within a range between 3.3 and 3.5, and the sodium silicate No. 5 has the molar ratio SiO₂/Na₂O within a range between 3.6 and 3.8. Among these, the sodium silicates No. 1 to No. 3 are also specified in JIS K1408. Any one or a mixture of the above-indicated sodium silicates may be used in the present invention. It is possible to control the molar ratio SiO₂/Na₂O by mixing a plurality of kinds of the above-indicated sodium silicates.

In the present invention, the sodium silicate used as the binder preferably has the molar ratio SiO₂/Na₂O not smaller than 1.0 and smaller than 3.0, and more preferably not smaller than 2.0 and smaller than 3.0, in order to obtain the coated sand which can fill the molding cavity with a particularly high filling density and which can give the casting mold having a high degree of the strength. Although the sodium silicate having the molar ratio SiO₂/Na₂O smaller than 2.0 is not commercially available, such a sodium silicate may be formed and used in the present invention. The sodium silicates having the molar ratio SiO₂/Na₂O not smaller than 2.0 and smaller than 3.0 are preferably used, since they are easily available and they give the coated sand having high degrees of fluidity and formability. Among the sodium silicates classified as described above, the sodium silicates Nos. 1 and 2 are advantageously used. The sodium silicates Nos. 1 and 2 give the coated sand having satisfactory filling properties and strength properties, with a high degree of stability, within a wide range of concentration of these sodium silicates in the aqueous solution of the water glass. The coated sand obtained by using, as the binder, the sodium silicate having the molar ratio SiO₂/Na₂O not smaller than 2.0 and smaller than 3.0 has the high degrees of fluidity and formability, but absorbs a larger amount of water than the coated sands obtained by using the other kinds of the sodium silicates. Therefore, the coated sand using the sodium silicate having the molar ratio SiO₂/Na₂O not smaller than 2.0 and smaller than 3.0 is preferably used for applications where the coated sand is used right after its production, and suitably used in dry environments such as in dry regions and cold regions. Further, it is recommended to store the coated sand in the absence of water.

The aqueous solution of the water glass used in the present invention is obtained by dissolving the water glass in water. A commercially available aqueous solution of the water glass is used as an undiluted solution, as purchased, or as a diluted solution obtained by adding water to the undiluted solution. A solid content in the aqueous solution, which is obtained by subtracting amounts of volatile substances such as the water and a solvent contained in the aqueous solution from an amount of the aqueous solution, is called a nonvolatile content and corresponds to an amount of the soluble silicate compound such as the sodium silicate described above. A higher ratio of the nonvolatile content (solid content) in the aqueous solution indicates a higher concentration of the water glass in the aqueous solution. Where the aqueous solution of the water glass consists solely of the undiluted solution, the nonvolatile content in the aqueous solution corresponds to an amount of a portion of the undiluted solution other than the water contained therein. On the other hand, where the diluted solution obtained by diluting the undiluted solution with the water is used as the aqueous solution of the water glass, the nonvolatile content in the aqueous solution corresponds to an amount of a portion of the aqueous solution other than the water contained in the undiluted solution and the water used to dilute the undiluted solution.

The nonvolatile content in the aqueous solution of the water glass is adequately selected depending on the kind of the water glass, for example, but preferably held within a range of 20-45% by mass. Where an adequate amount of a water glass component corresponding to the nonvolatile content is contained in the aqueous solution, the surfaces of the refractory aggregate can be evenly and uniformly coated with the water glass component, when the refractory aggregate and the aqueous solution of the water glass are mixed (kneaded) together. As a result, the casting mold having high degrees of flexural strength and hardness or resistance to scratching of its surface can be advantageously produced. Where an excessively small amount of the water glass component is contained in the aqueous solution of the water glass such that a total amount of the nonvolatile content is less than 20% by mass, it is necessary to dry the coated sand at a higher temperature for a longer period of time, so that there arises a problem of energy loss, for example. On the other hand, where the ratio of the nonvolatile content in the aqueous solution of the water glass is excessively high, it is difficult to uniformly coat the surfaces of the refractory aggregate with the water glass component, and a larger amount of the lumps are generated, giving rise to problems in improving properties of the casting mold. Therefore, the aqueous solution of the water glass is preferably prepared such that the nonvolatile content in the aqueous solution is not more than 45% by mass, and accordingly, a water content is not less than 55% by mass.

The coating layer of the water glass is formed on the surfaces of the refractory aggregate by using the aqueous solution of the water glass preferably in an amount of 0.1-2.5 parts by mass, and particularly advantageously in an amount of 0.2-2.0 parts by mass, in terms of the solid content or the nonvolatile content in the aqueous solution, per 100 parts by mass of the refractory aggregate. Here, the solid content in the aqueous solution of the water glass is measured in a manner described below: 10 g of a sample of the aqueous solution is weighed and put in a sample dish (a length: 90 mm; a width: 90 mm; a depth of 15 mm) formed of an aluminum foil; the sample dish is held on a heating plate whose temperature is held at 180±1° C., for 20 minutes; the sample dish is reversed upside down and held on the heating plate for 20 minutes; the sample dish is removed from the heating plate and cooled within a desiccator; then the sample is weighed. The solid content is calculated according to the following formula:

Solid content (%)=[Amount (g) of the sample after drying/Amount (g) of the sample before drying]×100

Where the aqueous solution of the water glass is used in an excessively small amount, it is difficult to effectively form the coating layer of the water glass on the surfaces of the refractory aggregate, so that it is difficult to sufficiently solidify or cure the obtained coated sand. On the other hand, where the aqueous solution of the water glass is used in an excessively large amount, an extra amount of the aqueous solution adherers to the surfaces of the refractory aggregate, so that it is difficult to uniformly form the coating layer, and there arises a risk of an increase of the amount of the lumps, an adverse influence on physical properties of the casting mold, and difficulty in removing the molding sand from a core after casting of a metal.

In the coated sand according to the present invention, the coating layer is formed on the surfaces of the refractory aggregate by using the aqueous solution of the water glass described above. However, the coating layer may contain suitable additives as necessary. The coating layer containing the additives is formed by: a method of initially mixing the suitable additives into the aqueous solution of the water glass, and then kneading or mixing the thus obtained mixture with the refractory aggregate; or a method of adding to the refractory aggregate, the suitable additives and the aqueous solution of the water glass separately from each other, and then uniformly kneading or mixing the thus obtained mixture.

Solid oxides and salts are advantageously used as the additives. The solid oxides and salts contained in the coating layer permit an advantageous improvement of a moisture resistance of the coated sand. It is effective to use the solid oxides such as oxides of silicon, zinc, magnesium, aluminum, calcium, lead and boron. Among these, silicon dioxide, zinc oxide, aluminum oxide and boron oxide are particularly preferably used. The silicon dioxide is preferably a precipitated silica or a pyrogenic silica. On the other hand, examples of the salts include silicofluoride salts, silicates, phosphates, borates, tetraborates and carbonates. Among these, zinc carbonate, potassium metaborate, sodium tetraborate and potassium tetraborate are preferably used. The above-indicated solid oxides and salts are used in an amount of not more than 100% by mass, and preferably about 0.5-5% by mass, with respect to the nonvolatile content in the aqueous solution of the water glass.

Further, it is effective to use, as other additives, coupling agents which strengthen a bond between the refractory aggregate and the water glass (binder). Examples of the coupling agents include silane coupling agents, zirconate coupling agents and titanate coupling agents. Also, it is effective to use lubricants which serve to improve the fluidity of the coated sand. Examples of the lubricants include: waxes such as paraffin wax, synthetic polyethylene wax and montanic acid wax; fatty acid amides such as stearic acid amide, oleic acid amide, and erucic acid amide; alkylene fatty acid amides such as methylenebis stearic acid amide and ethylenebis stearic acid amide; stearic acid; stearyl alcohol; metal stearate; lead stearate; zinc stearate; calcium stearate; magnesium stearate; monoglyceride stearate; stearyl stearate; and hydrogenated oils. Further, it is possible to use mold releasing agents such as paraffins, waxes, light oils, machine oils, spindle oils, insulating oils, waste oils, plant oils, fatty acid esters, organic acids, graphite particulates, mica, vermiculite, fluorine-based mold releasing agents, and silicone-based mold releasing agents. Each of the above-indicated additives other than the above-described solid oxides and salts is generally used in an amount of not more than 5% by mass, and preferably not more than 3% by mass, with respect to the nonvolatile content in the aqueous solution of the water glass.

The coated sand according to the present invention is produced by a method of uniformly kneading or mixing the aqueous solution of the water glass used as the binder and the additives used as necessary, with the heated refractory aggregate, such that the surfaces of the refractory aggregate are coated with the aqueous solution of the water glass and the water in the aqueous solution is evaporated, whereby the coated sand in the form of dry granules having fluidity at the room temperature is obtained. The water in the aqueous solution of the water glass (the coating layer) should be rapidly evaporated before solidification or curing of the water glass proceeds. Accordingly, in the present invention, the water in the aqueous solution of the water glass is evaporated within five minutes, and preferably within three minutes, after the aqueous solution is added to (mixed with) the refractory aggregate, to obtain the coated sand in the form of the dry granules. Where evaporation of the water takes an excessively long time, productivity of the coated sand is lowered due to an increase of a time required for the mixing (kneading) operation, and a risk of deactivation of the aqueous solution of the water glass is increased since the aqueous solution is exposed to CO₂ in the air for a longer period of time.

As effective means for rapidly evaporating the water in the aqueous solution of the water glass, the above-described method of producing the coated sand according to the present invention includes the steps of: preheating the refractory aggregate; and kneading or mixing the preheated refractory aggregate with the aqueous solution of the water glass. By kneading or mixing the aqueous solution of the water glass with the preheated refractory aggregate, the water in the aqueous solution can be extremely rapidly evaporated by the heat of the refractory aggregate, whereby the moisture percentage of the obtained coated sand is effectively reduced, so that the dry granules having fluidity at the room temperature are advantageously obtained. A temperature to which the refractory aggregate is preheated is adequately selected depending on the water content in the aqueous solution of the water glass and the amount of use of the aqueous solution, for example. It is desirable to preheat the refractory aggregate to a temperature of generally about 100-150° C., and preferably about 100-120° C. Where the preheating temperature of the refractory aggregate is excessively low, the water cannot be effectively evaporated, so that a time required for drying the coated sand is undesirably increased. Therefore, it is desirable to preheat the refractory aggregate to a temperature not lower than 100° C. On the other hand, where the refractory aggregate is preheated to an excessively high temperature, curing of the water glass proceeds while the obtained coated sand is cooled, and the composite particles are formed, so that the coated sand has problems in terms of its function, particularly in its strength or other physical properties.

In the coated sand obtained by the method described above, the amount of the lumps in the form of the composite particles is effectively reduced. Thus, it is possible to advantageously obtain the coated sand including not more than 3% by mass of the lumps whose particle diameter is larger than 20-mesh and which would be left on the 20-mesh screen when the coated sand was sieved with the screen.

The coated sand according to the present invention is produced as described above, such that the moisture percentage of the coated sand is controlled so as to be not more than 0.5% by mass, and preferably not more than 0.3% by mass, whereby the coated sand can more easily fill the molding cavity of the forming mold used for producing the casting mold, and the casting mold formed by using the coated sand is given excellent properties.

By using the thus obtained coated sand according to the present invention, the casting mold is produced by a method including the steps of: filling the molding cavity of the forming mold which gives the intended casting mold, with the coated sand; blowing a steam into the molding cavity such that the steam is passed through a filling phase of the coated sand; and holding the coated sand within the forming mold until the coated sand is dried and solidified or cured.

The forming mold such as a metallic forming mold or a wooden forming mold to be filled with the dry coated sand is preferably preheated and kept at an elevated temperature, to advantageously dry the coated sand moistened with the steam. The forming mold is generally preheated to and kept at a temperature of about 60-140° C., preferably about 80-130° C., and particularly preferably about 100-120° C. Where the forming mold is kept at an excessively high temperature, the steam does not sufficiently reach a surface of the filling phase of the coated sand filling the forming mold. On the other hand, where the forming mold is kept at an excessively low temperature, an undesirably long time is required for drying the formed casting mold. After the steam has been passed through the filling phase of the coated sand, the coated sand is preferably held within the forming mold for a predetermined period of time, before the obtained casting mold is removed from the forming mold. The coated sand is preferably held within the forming mold for 30-300 seconds, and more preferably for 30-180 seconds, after passing of the steam through the coated sand, such that the coated sand is dried while being held within the forming mold. The coated sand moistened with the steam has a high degree of thermal conductivity, so that by holding the coated sand within the preheated forming mold, the coated sand can be uniformly heated and solidified or cured. Additionally, the dry coated sand used to fill the forming mold is preferably preheated. Generally, where the forming mold is filled with the coated sand heated to a temperature not lower than 30° C., a flexural strength of the obtained casting mold can be advantageously improved. The coated sand is preferably heated to a temperature of about 30-100° C., and advantageously about 40-80° C.

After the preheated forming mold, specifically, its molding cavity has been filled with the dry coated sand, a pressurized steam is blown into the molding cavity through inlets provided in the forming mold, such that the steam is passed through the filling phase of the coated sand formed within the molding cavity, to moisten the filling phase and bond together particles of the coated sand, whereby a mass of the coated sand (a mass of the bonded particles of the coated sand) in the form of an integral casting mold is obtained. In the case where no additives are used, the water glass is generally solidified by evaporation of the water to dryness. On the other hand, where the oxides and the salts are used as a curing agent, the water glass is cured. In practical applications of the present invention, the curing agent is added, so that the filling phase of the coated sand is cured. However, the filling phase of the coated sand may be merely solidified. The steam may be a saturated steam or a superheated steam. The superheated steam is used in the state of a wet steam containing water drops. In the present invention, the superheated steam in the state of a dry steam which does not contain the water drops is not used to moisten the coated sand, but may be used to dry the coated sand.

A temperature of the steam blown into the molding cavity through the inlets in the forming mold and passed through the filling phase of the coated sand is generally held within a range of about 80-150° C., and preferably about 95-120° C. Particularly, the steam having a temperature around 100° C. is advantageously used, since the steam having an excessively high temperature requires a large amount of energy for its production. In the present invention, the steam is passed through the filling phase of the coated sand at a gauge pressure of about 0.01-0.3 MPa, and preferably about 0.01-0.1 MPa. In the case where the coated sand allows the steam to easily pass therethrough, the gauge pressure within the above-described ranges makes it possible to pass the steam through the entirety of the casting mold formed within the forming mold, and to reduce times required for passing of the steam and drying of the casting mold, and accordingly reduce a time required for formation of the casting mold. Further, the gauge pressure within the above-described ranges permits formation of the casting mold even in the case where the coated sand does not allow the steam to easily pass therethrough. An excessively high gauge pressure causes occurrence of staining around the inlets, while an excessively low gauge pressure gives rise to a risk that the steam cannot be passed through the entirety of the filling phase of the coated sand, so that the coated sand cannot be sufficiently moistened.

The steam is blown into the molding cavity through the inlets provided in the forming mold, and passed through the coated sand (filling phase) filling the molding cavity, as described above. A period for blowing the steam is adequately selected depending on the size of the forming mold and the number of the inlets, for example, so as to sufficiently moisten the water glass which covers the surfaces of the coated sand and serves as the binder, by blowing the steam to the surfaces of the coated sand, and accordingly to bond (bind) together the particles of the coated sand. The steam is generally blown into the molding cavity for a period of about 2-60 seconds. Where the period for blowing the steam is excessively short, it is difficult to sufficiently moisten the surfaces of the coated sand. On the other hand, where the period for blowing the steam is excessively long, there arises a risk of dissolution and discharge flow of the binder covering the surfaces of the coated sand. Passage of the steam through the coated sand filling the forming mold may be further improved by blowing the steam into the forming mold while sucking out an atmosphere within the forming mold through an exhaust vent provided in the forming mold. In the present invention, the method of moistening the coated sand is not particularly limited, but the above-described method of passing the steam through the coated sand is advantageously employed, from standpoints of the time required for forming the casting mold and simplicity of the process for forming the casting mold.

In the present invention, the steam may be blown into the molding cavity while a dry air, a heated dry air, a nitrogen gas or an argon gas is simultaneously blown into the molding cavity and passed through the filling phase of the coated sand, in order to actively dry the filling phase of the coated sand moistened with the steam. By blowing the steam into the molding cavity with the simultaneous blowing of the dry air or the like, the steam is easily blown throughout the molding cavity owing to the dry air or the like, so that uneven curing of the obtained casting mold can be avoided.

Further, in the present invention, the dry air, the heated dry air, the nitrogen gas or the argon gas is preferably blown into the molding cavity and passed through the filling phase of the casting mold, after the blowing of the steam into the molding cavity, in order to actively dry the filling phase of the coated sand moistened with the steam. By blowing the dry air, the heated dry air, the nitrogen gas or the argon gas into the molding cavity as described above, the filling phase of the coated sand is rapidly dried even in its central part, whereby curing or solidification of the filling phase is more advantageously accelerated to advantageously increase a curing rate of the filling phase. Further, the flexural strength or other properties of the obtained casting mold can be advantageously improved, and the time required for formation of the casting mold can be advantageously reduced. Blowing of the dry air or the like into the molding cavity is desirably carried out simultaneously with the blowing of the steam, and continued after termination of the blowing of the steam.

In the present invention, at least one of a carbon dioxide gas (a CO₂ gas), an ester gas, and a carbonate gas may be blown into the molding cavity, between a moment of initiation of the blowing of the steam and a moment of termination of the blowing of the dry air or the like. Solidification of the binder can be further accelerated by neutralizing the binder with the carbon dioxide gas, the ester gas, or the carbonate gas. Blowing of the carbon dioxide gas, the ester gas, or the carbonate gas may be carried out simultaneously with the blowing of the steam, or after termination of the blowing of the steam. Also, the blowing of the carbon dioxide gas, the ester gas, or the carbonate gas may be carried out simultaneously with the blowing of the dry air or the like, or with a time lag with respect to the blowing of the dry air or the like.

Further, before the steam is blown into the molding cavity, a pressure within the molding cavity may be reduced to a pressure preferably lower than the atmospheric pressure. For this purpose, a production machine of the casting mold may be provided with an apparatus for sucking the air from the molding cavity. By reducing the pressure within the molding cavity before the blowing of the steam, the steam can be more rapidly dispersed within the molding cavity, owing to the reduced pressure within the molding cavity.

The casting mold formed as described above may be heated with a micro wave to selectively evaporate the water only. If the water exists within the casting mold, the binder may be redissolved in the water, giving rise to a risk of deterioration of the flexural strength of the casting mold. Further, the water within the casting mold may be decomposed by the heat at the time of pouring of a molten metal, with a result of generation of a hydrogen gas, giving rise to an inherent problem of occurrence of a gas defect in the obtained cast product. Therefore, heating the formed casting mold with the micro wave to remove the water within the casting mold is an effective means for storage of the casting mold and an improvement of the quality of the cast product.

As the method of forming the casting mold by using the coated sand according to the present invention, it is possible to employ various known methods other than the above-described method of filling the forming mold with the coated sand. For example, it is possible to employ a multilayer molding method, specifically, a method of directly forming a three-dimensional casting mold by stacking layers of the coated sand, and curing a part of a stack of the layers of the coated sand, which part corresponds to the intended casting mold, as disclosed in JP-T-7-507508 and JP-A-9-141386, for example.

EXAMPLES

To clarify the present invention more specifically, some examples of the present invention will be described. However, it is to be understood that the present invention is by no means limited by the details of the illustrated examples. In the examples and comparative examples described below, “part” and “%” respectively indicate “part by mass” and “% by mass”, unless otherwise specified. Coated sands (CS) obtained in the examples and comparative examples were measured of their moisture percentage, amount of lumps, filling percentage and state of the sand, and casting molds obtained by using the respective coated sands were measured of their flexural strength and scratch hardness, while adhesion of sands to a surface of a kneading pot was observed, as described below.

—Measurement of the Moisture Percentage (%)—

2.0 g of each of the obtained coated sands (CS) was thrown into a flask of Karl Fischer Moisture Titrator (AQV-7 HIRANUMA AQUACOUNTER; available from Hiranuma Sanygo Co., Ltd., JAPAN) containing 100 ml of a dehydration solvent: AQUAMICRON ML (available from Mitsubishi Chemical Corporation, JAPAN) [Karl Fischer reagent (HYDRANAL Composite 5; available from Sigma-Aldrich Laborchemikalien Gmbh) was dropped into the flask in advance, to reduce the moisture amount to zero]. After stirring the mixture in the flask for several minutes with a magnetic stirrer, the HYDRANAL Composite 5 was dropped into the flask to determine a moisture amount and to calculate the moisture percentage from the determined value of the moisture amount.

—Measurement of the Amount (%) of the Lumps—

The CS obtained in each example was sieved with a 20-mesh screen to obtain composite particles (lumps) whose diameter is not smaller than 20-mesh and which were left on the screen. The amount of the lumps was obtained as a percentage value of the mass of the lumps with respect to the mass of the kneaded sand.

Amount (%) of the lumps=Mass of the lumps/[(Mass of the lumps)+(Mass of the sand not larger than 20-mesh)]×100

—Measurement of the Flexural Strength (kgf/cm²)—

By using each CS, a test piece having a width of 25.4 mm, a thickness of 25.4 mm and a length of 200 mm was formed, and measured of its breaking load by using a measuring device (a digital molding sand strength tester available from TAKACHIHO SEIKI CO., LTD., JAPAN). The flexural strength was calculated from the measured breaking load according to the following formula.

Flexural strength=1.5×LW/ab ²

[L: length (cm) of a support span, W: breaking load (kgf), a: width (cm) of the test piece, b: thickness (cm) of the test piece]

—Measurement of the Scratch Hardness (mm)—

The test piece formed by using each CS and having the width of 25.4 mm, the thickness of 25.4 mm and the length of 200 mm was measured of its scratch hardness by using a scratch hardness tester (GF type). Initially, a tooth provided at a distal end of the scratch hardness tester was pressed against a surface of the test piece. Then, a black lever provided in an upper portion of the tester was revolved one turn in the clockwise direction, and then revolved one turn in the counterclockwise direction. This set of operations of revolving the lever was repeated five more times, so that the tooth was gradually embedded into the test piece. A depth (mm) by which the tooth was embedded into the test piece was read on a scale provided on a side surface of the tester. A smaller depth indicates a higher degree of the scratch hardness of the test piece, while a larger depth indicates a lower degree of the scratch hardness.

—Measurement of the Filling Percentage (%)—

The filling percentage was calculated as a percentage value of a specific gravity (calculated by dividing a mass of the test piece by its volume) of the above-described test piece with respect to an absolute specific gravity of an aggregate.

Filling percentage (%)=[Mass (g) of the test piece/Volume (cm³) of the test piece]/Absolute specific gravity (g/cm³) of the aggregate×100

—Observation of Adhesion of the Sand to the Surface of the Kneading Pot—

After a kneading operation, conditions of adhesion of the sand to the surface of the kneading pot were examined by visually observing and rubbing the surface of the kneading pot. The conditions of adhesion were evaluated as “Good” where the sand does not adhere to the surface of the kneading pot, “Average” where the sand adheres to the surface of the kneading pot, but can be easily removed by rubbing, and “Poor” where the sand adheres to the surface of the kneading pot, and cannot be easily removed by rubbing.

Production Example 1 (Example 1) of the CS

A commercially available artificial molding sand LUNAMOS #50 (Trade Name; available from Kao Corporation, JAPAN) was provided as a refractory aggregate. An aqueous solution of a water glass was prepared by diluting commercially available sodium silicate No. 1 (Trade Name; available from Fuji Kagaku Corp., JAPAN) used as a binder, with water, such that the aqueous solution of the water glass has a nonvolatile content (an amount of a portion of the aqueous solution except the water contained therein) of 46.1%.

A Shinagawa-shiki universal stirrer (5DM-r type; manufactured by DALTON CO., LTD., JAPAN) was charged with the LUNAMOS #50 heated to a temperature of about 120° C., and the above-described aqueous solution of the water glass was introduced into the stirrer in an amount of 0.5 part, in terms of its nonvolatile content, with respect to 100 parts of the LUNAMOS #50. The contents in the stirrer were kneaded for three minutes to evaporate the water. After the contents were stirred and mixed until an aggregate structure of the sand particles collapsed, the contents were taken out of the stirrer, whereby a dry coated sand (CS) No. 1 having free flowing characteristics at the room temperature was obtained. The amount of the lumps included in the thus obtained CS and the moisture amount in the CS were measured. Further, the conditions of adhesion of the sand to the surface of the kneading pot were observed. Results of the measurements and observation are shown in a table given below.

Production Examples 2 to 9 (Examples 2 to 9) of the CS

CS Nos. 2 to 9 were obtained by the same procedure as in the Production Example 1, except that aqueous solutions of the water glass were prepared by diluting the commercially available sodium silicate No. 1 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solutions have the respective different nonvolatile contents of 44.0%, 41.6%, 39.7%, 37.5%, 33.5%, 30.0%, 25.0% and 20.0%.

Production Examples 10 to 15 (Examples 10 to 15) of the CS

CS Nos. 10 to 15 were obtained by the same procedure as in the Production Example 1, except that aqueous solutions of the water glass were prepared by diluting commercially available sodium silicate No. 2 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solutions have the respective different nonvolatile contents of 46.3%, 44.1%, 41.3%, 38.3%, 26.9% and 20.0%.

Production Examples 16 to 18 (Examples 16 to 18) of the CS

CS Nos. 16 to 18 were obtained by the same procedure as in the Production Example 1, except that aqueous solutions of the water glass were prepared by diluting commercially available sodium silicate No. 3 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solutions have the respective different nonvolatile contents of 37.5%, 25.6% and 12.8%.

Production Examples 19 to 21 (Examples 19 to 21) of the CS

CS Nos. 19 to 21 were obtained by the same procedure as in the Production Example 1, except that aqueous solutions of the water glass were prepared by diluting commercially available sodium silicate No. 5 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solutions have the respective different nonvolatile contents of 33.2%, 27.3% and 20.0%.

Production Example 22 (Example 22) of the CS

CS No. 22 was obtained by the same procedure as in the Production Example 1, except that a commercially available alumina-based spherical aggregate ESPEARL #60 (Trade Name; available from Yamakawa Sangyo Co., Ltd., JAPAN) was used as the refractory aggregate, and that the nonvolatile content in the aqueous solution of the water glass was 33.5%.

Production Example 23 (Example 23) of the CS

CS No. 23 was obtained by the same procedure as in the Production Example 1, except that MIKAWA KEISA No. 7 (Trade Name; available from Mikawa Keisa K.K., JAPAN) was used as the refractory aggregate, and that the water glass was used in an amount of 1.0 part, in terms of its nonvolatile content, with respect to 100 parts of the MIKAWA KEISA No. 7, while the nonvolatile content in the aqueous solution of the water glass was 33.5%.

Production Example 24 (Comparative Example 1) of the CS

CS No. 24 was obtained by the same procedure as in the Production Example 1, except that an aqueous solution of the water glass was prepared by diluting the commercially available sodium silicate No. 1 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solution has the nonvolatile content (the amount of the portion of the aqueous solution except the water contained therein) of 15.0%.

Production Example 25 (Comparative Example 2) of the CS

CS No. 25 was obtained by the same procedure as in the Production Example 1, except that an aqueous solution of the water glass was prepared by diluting the commercially available sodium silicate No. 2 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solution has the nonvolatile content (the amount of the portion of the aqueous solution except the water contained therein) of 13.3%.

Production Example 26 (Comparative Example 3) of the CS

CS No. 26 was obtained by the same procedure as in the Production Example 1, except that an aqueous solution of the water glass was prepared by diluting the commercially available sodium silicate No. 3 (Trade Name; available from Fuji Kagaku Corp.) used as the binder, with the water, such that the aqueous solution has the nonvolatile content (the amount of the portion of the aqueous solution except the water contained therein) of 9.6%.

Production Example 27 (Comparative Example 4) of the CS

CS No. 27 was obtained by the same procedure as in the Production Example 1, except that the commercially available sodium silicate No. 5 (Trade Name; available from Fuji Kagaku Corp.) used as the binder was diluted with the water, such that the thus prepared aqueous solution has the nonvolatile content (the amount of the portion of the aqueous solution except the water contained therein) of 15.0%.

—Production Example of the Casting Mold—

Each of the CS Nos. 1 to 27 obtained in the above-described Production Examples and having a temperature of 20° C. was blown into a forming mold heated to 110° C., at a gauge pressure of 0.3 MPa, such that the forming mold was filled with the CS. Then, a steam having a temperature of 99° C. was blown into the forming mold, at a gauge pressure of 0.05 MPa for five seconds, such that the steam was passed through a filling phase of the coated sand filling the forming mold. After blowing of the steam was terminated, a hot air having a temperature of 150° C. was blown into the forming mold, at a gauge pressure of 0.03 MPa for two minutes, to cure the CS filling the forming mold. The thus produced casting mold was used as a test piece (25.4 mm×25.4 mm×200 mm).

—Measurement of the Casting Mold—

The thus obtained test pieces produced by using the respective CS Nos. 1 to 27 were measured of their filling percentage, flexural strength and scratch hardness, according to the methods described above. Results of the measurements are shown in Tables 1 to 3 given below.

TABLE 1 Example Example Example Example Example Example Example Example Example 1 2 3 4 5 6 7 8 9 Refractory LUNAMOS #50 100 100 100 100 100 100 100 100 100 Aggregate Aqueous Sodium Silicate 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 Solution of No. 1 Water Glass Nonvolatile Content (%) of Water Glass 46.1 44.0 41.6 39.7 37.5 33.5 30.0 25.0 20.0 Conditions of Adhesion of Sand to Good Good Good Good Good Good Good Good Average Kneading Pot Properties Moisture Percentage (%) <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 0.2 of Sand Amount (%) of Lumps 1.37 0.40 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 <0.1 Filling Percentage (%) 52.4 53.5 56.1 57.0 56.5 56.2 56.1 56.7 57.2 State of Sand Dry Dry Dry Dry Dry Dry Dry Dry Dry Properties of Flexural Strength (kgf/cm²) 27.0 35.7 58.4 56.7 63.3 62.7 57.5 56.8 57.8 Casing Mold Scratch Hardness (mm) 6.5 6.0 5.6 4.5 3.7 3.3 3.3 3.3 3.0

TABLE 2 Example Example Example Example Example Example Example Example Example 10 11 12 13 14 15 16 17 18 Refractory LUNAMOS #50 100 100 100 100 100 100 100 100 100 Aggregate Aqueous Sodium Silicate 0.5 0.5 0.5 0.5 0.5 0.5 — — — Solution of No. 2 Water Glass Sodium Silicate — — — — — — 0.5 0.5 0.5 No.3 Nonvolatile Content (%) of Water Glass 46.3 44.1 41.3 38.3 26.9 20.0 37.5 25.6 12.8 Conditions of Adhesion of Sand to Good Good Good Good Good Average Good Good Average Kneading Pot Properties Moisture Percentage (%) <0.1 <0.1 <0.1 <0.1 <0.1 0.2 <0.1 <0.1 0.33 of Sand Amount (%) of Lumps 3.62 1.50 1.02 0.13 0.24 <0.1 3.50 0.30 0.14 Filling Percentage (%) 52.9 53.5 54.7 54.5 56.7 57.3 48.7 56.8 57.6 State of Sand Dry Dry Dry Dry Dry Dry Dry Dry Dry Properties of Flexural Strength (kgf/cm²) 15.1 21.6 38.4 40.2 59.3 50.3 15.7 54.5 53.1 Casting Mold Scratch Hardness (mm) 6.2 6.0 5.8 4.9 3.5 3.1 5.1 1.7 0.9

TABLE 3 Comparative Comparative Comparative Comparative Example Example Example Example Example Example Example Example Example 19 20 21 22 23 1 2 3 4 Refractory LUNAMOS #50 100 100 100 — — 100 100 100 100 Aggregate ESPEARL #60 — — — 100 — — — — — MIKAWA KEISA No. 7 — — — — 100 — — — — Aqueous Sodium Silicate No. 1 — — — 0.5 1.0 0.5 — — — Solution of Sodium Silicate No. 2 — — — — — — 0.5 — — Water Glass Sodium Silicate No. 3 — — — — — — — 0.5 — Sodium Silicate No. 5 0.5 0.5 0.5 — — — — — 0.5 Nonvolatile Content (%) of Water Glass 33.2 27.3 20.0 33.5 33.5 15.0 13.3 9.6 15.0 Conditions of Adhesion of Sand to Good Good Average Good Good Poor Poor Poor Poor Kneading Pot Properties Moisture Percentage (%) <0.1 <0.1 0.35 <0.1 <0.1 0.85 0.74 1.39 1.31 of Sand Amount (%) of Lumps 1.9 0.1 0.21 <0.1 <0.1 Could not be Could not be Could not be Could not be measured measured measured measured Filling Percentage (%) 54.4 55.5 56.7 54.2 48.1 Could not be Could not be Could not be Could not be measured measured measured measured State of Sand Dry Dry Dry Dry Dry Wet Wet Wet Wet Properties of Flexural Strength (kgf/cm²) 15.2 19.5 16.2 68.9 15.8 Could not be Could not be Could not be Could not be Casting Mold measured measured measured measured Scratch Hardness (mm) >7.0 5.9 4.5 2.4 5.7 Could not be Could not be Could not be Could not be measured measured measured measured

As is apparent from the results shown in Tables 1 to 3, it is recognized that the CS Nos. 24 to 27 which were obtained in the Comparative Examples 1 to 4 in the wet state and whose moisture percentages are not less than 0.5% could not satisfactorily fill the forming mold, whereas the CS Nos. 1 to 23 obtained in the Examples 1 to 23 and having the moisture percentages of not more than 0.5% have sufficiently high degrees of fluidity. Also, it is recognized that the use of the sodium silicate Nos. 1 and 2 having SiO₂/Na₂O molar ratios not smaller than 2.0 and smaller than 3.0 results in high degrees of the filling percentage and the flexural strength, within a wide range of the nonvolatile content in the aqueous solution of the water glass. Accordingly, by using the sodium silicate Nos. 1 and 2, the coated sand can be produced with a high degree of freedom of choice of its composition, and the obtained coated sand has a high degree of formability. On the other hand, where the sodium silicate Nos. 3 to 5 having the SiO₂/Na₂O molar ratios within a range between 3.0 and 4.0 are used, as in the Examples 16 to 21, sufficiently high degrees of the physical properties and formability are achieved only within a narrow range of the nonvolatile content in the aqueous solution of the water glass. Accordingly, it is understood that the use of the sodium silicate Nos. 3 to 5 results in reduction of the freedom of choice of the composition of the coated sand. Further, it is recognized that particularly higher degrees of the filling percentage, flexural strength and scratch hardness can be achieved in the case where the coated sands were produced by using the aqueous solutions of the water glass prepared by using the sodium silicate Nos. 1 and 2 such that the aqueous solutions have 20-45% by mass of the nonvolatile content. In this respect, it is noted that undiluted solutions of the sodium silicate Nos. 3 to 5 have not less than about 30% and less than about 40% of the nonvolatile content, so that no experiment was conducted on aqueous solutions of the sodium silicate Nos. 3 to 5 having the nonvolatile content more than the above-indicated range. 

1. A coated sand which is in a dry state and which has fluidity at the room temperature, the coated sand being obtained by mixing an aqueous solution of a water glass used as a binder, with a heated refractory aggregate, to evaporate water in the aqueous solution of the water glass, for thereby forming a coating layer of the binder on surfaces of the refractory aggregate, the coated sand being characterized in that its moisture percentage is controlled so as to be not more than 0.5% by mass.
 2. The coated sand according to claim 1, characterized in that the coated sand includes not more than 3% by mass of lumps which do not pass through a 20-mesh screen.
 3. The coated sand according to claim 1, characterized in that the aqueous solution of the water glass contains an alkali metal silicate as its major component.
 4. The coated sand according to claim 3, characterized in that the alkali metal silicate has a molar ratio of silicon dioxide to an alkali metal oxide, which molar ratio is not smaller than 1.0 and smaller than 3.0.
 5. The coated sand according to claim 3, characterized in that the alkali metal silicate is sodium silicate.
 6. The coated sand according to claim 5, characterized in that the sodium silicate has a molar ratio SiO₂/Na₂O of not smaller than 1.0 and smaller than 3.0.
 7. The coated sand according to claim 1, characterized in that a nonvolatile content in the aqueous solution of the water glass is 20-45% by mass.
 8. The coated sand according to claim 1, characterized in that the aqueous solution of the water glass is used in an amount of 0.1-2.5 parts by mass, in terms of its solid content, with respect to 100 parts by mass of the refractory aggregate.
 9. A method of producing the coated sand according to claim 1, characterized in that the refractory aggregate and the aqueous solution of the water glass are mixed together such that the water in the aqueous solution of the water glass is evaporated within 5 minutes after addition of the aqueous solution of the water glass to the refractory aggregate, to obtain the coated sand having the moisture percentage of not more than 0.5% by mass.
 10. A method of producing a casting mold, characterized in that the casting mold is obtained by filling a molding cavity of a forming mold which gives the casting mold, with the coated sand according to claim 1, and then passing a steam through the coated sand, to solidify or cure the coated sand within the forming mold.
 11. The method of producing the casting mold according to claim 10, characterized in that a dry air, a heated dry air, a nitrogen gas or an argon gas is further passed through a filling phase of the coated sand filling the molding cavity of the forming mold, simultaneously with or after passing the steam through the coated sand.
 12. The method of producing the casting mold according to claim 10, characterized in that at least one of a carbon dioxide gas, an ester gas and a carbonate gas is passed through a filling phase of the coated sand filling the molding cavity of the forming mold, simultaneously with or after passing the steam through the coated sand.
 13. The method of producing the casting mold according to claim 10, characterized in that a pressure within the molding cavity of the forming mold is reduced before passing the steam through the coated sand.
 14. The method of producing the casting mold according to claim 10, characterized in that the coated sand is preheated to a temperature not lower than 30° C., and then the molding cavity of the forming mold is filled with the preheated coated sand.
 15. The method of producing the casting mold according to claim 10, characterized in that the forming mold is preheated and kept at an elevated temperature.
 16. A method of producing a casting mold, characterized in that the casting mold is formed by multilayer molding using the coated sand according to claim
 1. 